Microsatellites, or STRs, represent the most discriminating genetic marker used in forensic genetics to identify traces and persons. They are helpful to analyze samples containing the DNA of one person or the DNA of two persons occurring in comparable quantities. This is why those genetic markers have been retained to feed national DNA databases used to fight against criminals all around the world. For instance, the Swiss DNA database, launched in 2000, contains presently more than 140,000 DNA profiles. One limitation of STRs is that they cannot resolve unbalanced mixtures. In particular, the minor contributor of a DNA mixture cannot be detected using conventional DNA markers (STR, SNP, Sanger sequencing, etc.) when it represents less than about 10% of the major contributor. Below this threshold, the DNA profile of the minor contributor, who is potentially the perpetrator of the crime, can be masked, preventing his genetic identification.
Such situation is frequent and may happens for instance when analyzing touched object (e.g. samples from the steering-wheel of a stolen cars, screw-drivers, door handles, etc) or samples from rape cases (e.g. gynecological swabs). A way to avoid this limitation is to use STRs located on the Y chromosome. Those sex-specific markers can detect a few male DNA within a high background of female DNA. However, Y-STRs are of limited information mainly for three reasons. First, they are helpful to analyze only mixtures of male and female DNA when the male is the minor contributor and not other types of mixtures. Second, they do not discriminate among paternally related males and therefore allow to identify a lineage. Third, some Y-markers are frequent in the population compromising the discrimination of different male lineages and reducing the power of the DNA evidence.
In view of the above, there is a need to develop an improved method to overcome the identified limits and enabling to identify an individual, rather than a lineage.